pixman-renderer: Don't unreference NULL hw_buffer image
If the buffer for an output was never set, or was reset to NULL at some
point, destroying the renderer output private would cause a crash.
diff --git a/src/pixman-renderer.c b/src/pixman-renderer.c
index 77ae99e..8d95197 100644
--- a/src/pixman-renderer.c
+++ b/src/pixman-renderer.c
@@ -507,7 +507,9 @@
struct pixman_output_state *po = get_output_state(output);
pixman_image_unref(po->shadow_image);
- pixman_image_unref(po->hw_buffer);
+
+ if (po->hw_buffer)
+ pixman_image_unref(po->hw_buffer);
po->shadow_image = NULL;
po->hw_buffer = NULL;